FIG. 1 (prior art) is a simplified block diagram of a so-called magnetic stripe reader 1 as might be part of a common point of sale credit card terminal device. A card 3 encoded with a magnetic stripe 20, is physically swiped past a magnetic pick-up unit 8, generating a magnetic track signal 10. Typically, a F/2F waveform generator 2 then processes this magnetic track signal 10. This may occur by first amplifying the small amplitude magnetic track signal 10 using a gain stage 4 to generate an amplified track signal 11, then filtering the amplified track signal 11 using a low-pass filter 5 resulting in an amplified, filtered track signal 12. This signal is then processed by a peak detector 6 to produce a digital signal 13 that reflects the magnetic stripe 20 encoded on the card 3.
FIG. 2 (prior art) further details the signal processing steps performed by a magnetic stripe reader illustrated in FIG. 1. A typical card 3 is encoded with a magnetic stripe 20. This magnetic stripe is a series of magnetic pole pairs disposed end-to-end such that magnetic flux concentrations are linearly spaced along the magnetic stripe 20. A typical magnetic stripe 20 is linearly subdivided into a series of equal length bit cells 26 that represent a digital bit string 23. If one magnetic pole-pair is encoded across a single bit cell, this represents a zero bit 25. If two magnetic pole-pairs are encoded end-to-end across a single bit cell, this represents a one bit 24. Viewed over time, the signal is a sequential superposition of signals of a fixed frequency representing a zero bit and of signals of twice the fixed frequency representing a one bit. For this reason, the digital waveform 22 that results from reading typical magnetic cards is commonly termed a F/2F waveform. Furthermore, the elements used to transform a magnetic track signal into digital waveform 22 may be termed a F/2F waveform generator.
FIG. 3 (prior art) represents a first approach to F/2F waveform generation in magnetic stripe readers. The magnetic track signal from the magnetic pick-up unit 31 is amplified and low-pass filtered before the signal is digitized by an analog-to-digital converter 36. The digital signal may be further filtered by a digital filter 37 before being processed by a digital peak detector 39 to generate the F/2F waveform. The digital approach to F/2F waveform generation has several disadvantages. It is large and complex to implement on silicon, leading to high production cost. Furthermore, both the digital filtering and peak detection schemes require a significant and undesirably expensive software implementation effort.
FIG. 4 (prior art) illustrates a second approach to F/2F waveform generation in magnetic stripe readers. The circuit switches between digital high and digital low output values when the magnetic track signal crosses predetermined threshold voltages. The threshold voltages are determined by the gain and operation of analog circuit components.